Gas insulated metal-enclosed transmission line (GIL) can improve the flexibility on the choice of the UHVDC transmission line corridors. Therefore, theresearch of SF_6gas replacement is of great significance. In this paper, under negative DC voltage and negative lightning impulse voltage, the experiment research of SF_6, SF_6/N_2 and CF_3I/N_2 gases breakdown characteristic were applied within the scope of 0.4~0.7MPa gas pressure. Under the samecondition of the gas, the corresponding cylindrical insulator flashover characteristics had been studied. The results show that with gas pressure increasing, SF_6, SF_6/N_2 and CF_3I/N_2 gas breakdown voltage presented the nearly linear growth trend,and in the same gas pressure, the breakdown field strength of SF_6 is maximal, SF_6/N_2 is second, CF_3I/N_2is minimal. The insulator flashover voltage in CF_3I/N_2gas mixture is far lower than it in the SF_6/N_2 and SF_6gas. The results indicate that with 0.7MPa gas pressure, and mixture ratio of 2:8, SF_6/N_2 gas insulation performance and SF_6 gas insulation performance with 0.5MPa gas pressure is almost equal, and GWP (global warming potential)can be reduced about 80%.As a consequence,the SF_6/N_2 gaswhich the pressure is 0.7 MPaand the mixture ratio is 2:8can be applied to HVDC-GIL. And compared with SF_6/N_2 and SF_6 gas, CF_3I/N_2 gas mixture is not suitable for high pressure gas insulated metal-enclosed equipmentbecause of the low gas insulation performance.